Leif Hildingsson

Fuel Octane Effects in the Partially Premixed Combustion Regime in Compression Ignition Engines

Previous work has showed that it may be advantageous to use fuels of lower cetane numbers compared to today’s diesel fuels in compression ignition engines. The benefits come from the longer ignition delays that these fuels have. There is more time available for the fuel and air to mix before combustion starts which is favourable for achieving low emissions of NOx and smoke though premixing usually leads to higher emissions of CO and unburned hydrocarbons. In the present work, operation of a single-cylinder lightduty compression ignition engine on four different fuels of different octane numbers, in the gasoline boiling range, is compared to running on a diesel fuel. The gasoline fuels have research octane numbers (RON) of 91, 84, 78, and 72. These are compared at a low load/low speed condition (4 bar IMEP / 1200 rpm) in SOI sweeps as well as at a higher load and speeds (10 bar IMEP / 2000 and 3000 rpm) in EGR sweeps. There is a NOx advantage for the 91 RON and 84 RON fuels at the lower load. At the higher load, NOx levels can be reduced by increasing EGR for all gasolines while maintaining much lower smoke levels compared to the diesel. In the conditions studied, the optimum RON range might be between 75 and 85.

Low NOx and Low Smoke Operation of a Diesel Engine Using Gasoline-Like Fuels

Much of the technology in advanced diesel engines, such as high injection pressures, is aimed at overcoming the short ignition delay of conventional diesel fuels to promote premixed combustion in order to reduce NOx and smoke. Previous work in a 2 litre single cylinder diesel engine with a compression ratio of 14 has demonstrated that gasoline fuel, because of its high ignition delay, is very beneficial for premixed compression ignition compared to a conventional diesel fuel. We have now done similar studies in a smaller — 0.537 litre — single cylinder diesel engine with a compression ratio of 15.8. The engine was run on three fuels of very different auto-ignition quality — a typical European diesel fuel with a cetane number (CN) of 56, a typical European gasoline of 95 RON and 85 MON with an estimated CN of 16 and another gasoline of 84 RON and 78 MON (estimated CN of 21). The previous results with gasoline were obtained only at 1200 rpm — here we compare the fuels also at 2000 rpm and 3000 rpm. At 1200 rpm, at low loads (∼4 bar IMEP) when smoke is negligible, NOx levels below 0.4 g/kWh can be easily attained with gasoline without using EGR while this is not possible with the 56 CN European diesel. At these loads, the maximum pressure rise rate is also significantly lower for gasoline. At 2000 rpm, with 2 bar absolute intake pressure, NOx can be reduced below 0.4 g/kWh with negligible smoke (FSN <0.1) with gasoline between 10 and 12 bar IMEP using sufficient EGR while this is not possible with the diesel fuel. At 3000 rpm, with the intake pressure at 2.4 bar absolute, NOx of 0.4 g/KWh with negligible smoke was attainable with gasoline at 13 bar IMEP. Hydrocarbon and CO emissions are higher for gasoline and will require after-treatment. High peak heat release rates can be alleviated using multiple injections. Large amounts of gasoline, unlike diesel, can be injected very early in the cycle without causing heat release during the compression stroke and this enables the heat release profile to be shaped.Copyright

Simultaneous OH- and Formaldehyde-LIF Measurements in an HCCI Engine

Simultaneous OH- and formaldehyde LIF measurements have been performed in an HCCI engine using two laser sources working on 283 and 355 nm, respectively. Two ICCD camera systems, equipped with long-pass filters, were used to collect the LIF signals. The simultaneous images of OH and formaldehyde were compared with heat-release calculated from the pressure-trace matching the cycle for the LIF measurements. The measurements were performed on a 0.5-l, single-cylinder optical engine equipped with port-fuel injection system. A blend of iso-octane and n-heptane was used as fuel and the compression ratio was set to 12:1. The width of the laser sheet was 40 mm and hence covered approximately half of the cylinder bore. At some 20 CAD BTDC low temperature reactions are present and formaldehyde is formed. The formaldehyde signal is then rather constant until the main heat-release starts just before TDC, where the signal decreases rapidly to low values. From some 15 CAD to 5 CAD BTDC the formaldehyde is uniformly distributed in the imaged area. As formaldehyde decreases, OH increases and follows the main rate of heat release curve, though with a slight lag in phase. Thereafter OH is formed in the areas from which the formaldehyde has disappeared and the OH signal is present to some 20 CAD ATDC. (Less)

Low NOx and Low Smoke Operation of a Diesel Engine Using Gasolinelike Fuels

Much of the technology in advanced diesel engines, such as high injection pressures, is aimed at overcoming the short ignition delay of conventional diesel fuels to promote premixed combustion in order to reduce NOx and smoke. Previous work in a 2 l single-cylinder diesel engine with a compression ratio of 14 has demonstrated that gasoline fuel, because of its high ignition delay, is very beneficial for premixed compression-ignition compared with a conventional diesel fuel. We have now done similar studies in a smaller-0.537 l-single-cylinder diesel engine with a compression ratio of 15.8. The engine was run on three fuels of very different auto-ignition quality-a typical European diesel fuel with a cetane number (CN) of 56, a typical European gasoline of 95 RON and 85 MON with an estimated CN of 16 and another gasoline of 84 RON and 78 MON (estimated CN of 21). The previous results with gasoline were obtained only at 1200 rpm-here we compare the fuels also at 2000 rpm and 3000 rpm. At 1200 rpm, at low loads (similar to 4 bars indicated mean effective pressure (IMEP)) when smoke is negligible, NOx levels below 0.4 g/kWh can be easily attained with gasoline without using exhaust gas recirculation (EGR), while this is not possible with the 56 CN European diesel. At these loads, the maximum pressure-rise rate is also significantly lower for gasoline. At 2000 rpm, with 2 bars absolute intake pressure, NOx can be reduced below 0.4 g/kW h with negligible smoke (FSN < 0.1) with gasoline between 10 bars and 12 bars IMEP using sufficient EGR, while this is not possible with the diesel fuel. At 3000 rpm, with the intake pressure at 2.4 bars absolute, NOx of 0.4 g/kW h with negligible smoke was attainable with gasoline at 13 bars IMEP. Hydrocarbon and CO emissions are higher for gasoline and will require after-treatment. High peak heat release rates can be alleviated using multiple injections. Large amounts of gasoline, unlike diesel, can be injected very early in the cycle without causing heat release during the compression stroke and this enables the heat release profile to be shaped. [DOI: 10.1115/1.4000602] (Less)

Numerical and Experimental Investigation of Turbulent Flows in a Diesel Engine

This paper presents a study of the turbulence field in an optical diesel engine operated under motored conditions using both large eddy simulation (LES) and Particle Image Velocimetry (PIV). The study was performed in a laboratory optical diesel engine based on a recent production engine from VOLVO Car. PIV is used to study the flow field in the cylinder, particularly inside the piston bowl that is also optical accessible. LES is used to investigate in detail the structure of the turbulence, the vortex cores, and the temperature field in the entire engine, all within a single engine cycle. The LES results are compared with the PIV measurements in a 40 x 28 mm domain ranging from the nozzle tip to the cylinder wall. The LES grid consists of 1283 cells. The grid dynamically adjusts itself as the piston moves in the cylinder so that the engine cylinder, including the piston bowl, is described by the grid. In the intake phase the large-scale swirling and tumbling flow streams are shown to be responsible for the generation of large-scale vortex pipes which break down to small-scale turbulent eddies. In the later phase of compression turbulence is mainly produced in the engine bowl. The bore wall and the piston bowl wall heat the fluid near the walls. Turbulence and the large-scale coherent vortex shedding due to the Kelvin-Helmholtz instability are responsible for the enhanced heat transfer between the bulk flow and the walls. A temperature inhomogeneity of about 50 - 60 K can be generated in the cylinder. (Less)

Formaldehyde and Hydroxyl Radicals in an HCCI Engine - Calculations and LIF-Measurements

Concentrations of hydroxyl radicals and formaldehyde were calculated using homogeneous (HRM) and stochastic reactor models (SRM), and the result was compared to LIF measurements from an optically accessed iso-octane/n-heptane-fuelled homogeneous charge compression ignition (HCCI) engine. The comparison was at first conducted from averaged total concentrations/signal strengths over the entire combustion volume, which showed a good qualitative agreement between experiments and calculations. Time- and the calculation-inlet-temperature-resolved concentrations of formaldehyde and hydroxyl radicals obtained through HRM are presented. Probability density plots (PDPs) through SRM calculations and LIF measurements are presented and compared, showing a very good agreement considering their delicate and sensitive nature. Thus it is concluded that SRM is a valid model for these purposes, justifying the use of SRM in order to extend the evaluated concentration ranges of the analyzed species beyond the detection/separation level. It is shown that formaldehyde concentration increases slowly, contrary to hydroxyl which is fast developed. Formaldehyde is locally fast consumed once high temperature chemistry has started, and the highest maximum concentrations of formaldehyde are found in cases where low-temperature chemistry was never transitioned to high-temperature ignition. The PDPs from SRM calculations give increased insight of the occurrence and development of autoignition. During the onset of ignition, the regions with the highest formaldehyde concentrations also have the highest concentrations of hydroxyl radicals. The low-temperature heat release (LTHR) maximum occurs before maximum of formaldehyde, and the regions of (for the LTHR regime relatively) high hydroxyl concentrations gradually becomes fewer until they cease to exist; this occurs after the LTHR peak but before formaldehyde maximum. During the transition state all regions have similar formaldehyde concentrations but varying concentrations of hydroxyl. (Less)

Effect of Turbulence and Initial Temperature Inhomogeneity on Homogeneous Charge Compression Ignition Combustion

A 0.5-liter optical HCCI engine firing a mixture of n-heptane (50%) and iso-octane (50%) with air/fuel ratio of 3 is studied using large eddy simulation (LES) and laser diagnostics. Formaldehyde and OH LIF and in-cylinder pressure were measured in the experiments to characterize the ignition process. The LES made use of a detailed chemical kinetic mechanism that consists of 233 species and 2019 reactions. The auto-ignition simulation is coupled with LES by the use of a renormalized reaction progress variable. Systematic LES study on the effect of initial temperature inhomogeneity and turbulence intensity has been carried out to delineate their effect on the ignition process. It was shown that the charge under the present experimental condition would not be ignited without initial temperature inhomogeneity. Increasing temperature inhomogeneity leads to earlier ignition whereas increasing turbulence intensity would retard the ignition. This is mostly due to the effect of turbulence on the bulk flow that turbulence tends to decrease the temperature inhomogeneity by enhanced eddy heat transfer. The LES results suggest that desirable ignition timing could be achieved by controlling the turbulence intensity and temperature inhomogeneity.

Optical Diagnostics of HCCI and UNIBUS Using 2-D PLIF of OH and Formaldehyde

Simultaneous OH- and formaldehyde planar-LIF measurements have been performed in an optical engine using two laser sources working on 283 and 355 nm, respectively. The measurements were performed in a light- duty diesel engine, using n-heptane as fuel, converted to single- cylinder operation and modified for optical access. It was also equipped with a direct-injection, common-rail system as well as an EGR system. The engine was operated in both HCCI mode, using a single fuel injection, and UNIBUS (Uniform Bulky Combustion System) mode, using two injections of fuel with one of the injections at 50 CAD before TDC and the other one just before TDC. The OH and formaldehyde LIF images were compared with the heat- release calculated from the pressure-traces. Analyses of the emissions, for example NOx and HC, were also performed for the different operating conditions. (Less)

Simultaneous PLIF Measurements for Visualization of Formaldehyde- and Fuel- Distributions in a DI HCCI Engine

Simultaneous laser-induced fluorescence (LIF) imaging of formaldehyde and a fuel-tracer have been performed in a direct-injection HCCI engine. A mix of N-heptane and iso-octane was used as fuel and Toluene as fluorescent tracer. The experimental setup involves two pulsed Nd:YAG lasers and two ICCD cameras. Frequency-quadrupled laser radiation at 266 nm from one of the Nd:YAG lasers was used for excitation of the fuel tracer. The resulting fluorescence was detected with one of the ICCD cameras in the spectral region 270-320 nm. The second laser system provided frequency-tripled radiation at 355 nm for excitation of formaldehyde. Detection in the range 395-500 nm was achieved with the second ICCD. The aim of the presented work is to investigate the applicability of utilizing formaldehyde as a naturally occurring fuel marker. Formaldehyde is formed in the low-temperature reactions (LTR) prior to the main combustion and should thus be present were fuel is located until it is consumed. Measurements were performed when injecting fuel early and late in the compression stroke. Early injection timing results in a homogeneous charge at the time of auto-ignition, while late timing gives a more stratified charge. The crank angle position at which measurements were performed was altered to cover the entire combustion cycle. The measurement images show instantaneous distributions of toluene and formaldehyde respectively. Images from both early and late injection and at all crank angle degrees show good spatial resemblance between toluene signal area and formaldehyde signal area. The work presented in this paper shows that formaldehyde is a feasible alternative to traditional fuel tracers for visualizing fuels featuring low-temperature reactions in HCCI combustion

Optical Diagnostics of Hcci and Low-Temperature Diesel Using Simultaneous 2-D Plif of Oh and Formaldehyde

Simultaneous OH- and formaldehyde planar-LIF measurements have been performed in an optical engine using two laser sources working on 283 and 355 nm, respectively. The engine used for the measurements was a car diesel engine converted to single-cylinder operation and modified for optical access. The fuel, n-heptane, was injected by a direct injection common-rail system and the engine was also fitted with an EGR system. The engine was operated in both HCCI mode and diesel mode. Due to the low load, the diesel mode resulted in low-temperature diesel combustion and because of limitations in maximum pressure and maximum rate of pressure increase of the optical engine, the diesel mode was run at a higher EGR percentage than the HCCI mode to slow down the combustion. A third mode, pilot combustion, was also investigated. This pilot combustion is created by an injection at 30 CAD before TDC followed by a second injection just before TDC. The OH and formaldehyde LIF images were compared with the heat-release calculated from the pressure traces. Analyses of the emissions, of for example NOx and HC, were also performed for the different operating modes. (Less)